Here's what the actor had to say about the matter:
"The suit was designed by an artist named Phil Saunders who collaborated with Adi Granov based on the comic art of Granov. It evolved over many months and then was brought to Winston Studios to refine the design in order to build it as a working practical costume. Winston modeled and then fabricated the actual suit. It is difficult to identify a single person as the designer because they each contributed to the final design. The EW (Entertainment Weekly) article correctly stated, however, that the suit was built by Winston. I hope this helps."
Iron Man will be released on May 2, 2008.